In Dutch oven cook spaghetti according to package directions. Add asparagus the last 1 minute of cooking. Drain. Return to pan.

Meanwhile, in large skillet melt butter and cook mushrooms and sweet peppers over medium heat for 8 to 10 minutes or until mushrooms are tender, stirring occasionally. Stir in flour and black pepper until well combined. Add broth and milk all at once. Cook and stir until thickened and bubbly.